Power tools of the aforementioned type are known from DE 10 2006 052 808 A1. In this case, they are designed as jigsaws, wherein the saw blade is driven in a stroke motion and, in the case of exclusively stroke motion drive, operates in the standard working mode, i.e. without further rotary and pivoting motions, with the plane of the saw blade aligned in the longitudinal direction of the machine. If a pivoting motion, about a pivot axis lying transversely in relation to the plane of the saw blade, is superposed on the stroke motion drive, the jigsaw operates in the pendulum stroke mode. A further operating mode is obtained if the jigsaw is to be operated as a so-called “scrolling jigsaw”, in which case, in addition to the stroke drive of the saw blade, there is obtained an ability to rotate about a rotary axis extending in the direction of its stroke axis, such that the respective working direction can be set substantially through the rotation of the saw blade.
If this is effected with the aid of a detection system, by means of which a working line predefined on the workpiece side can be acquired, the acquired data can be used to control a positioning device, by means of which the saw blade can be moved about its rotary axis and therefore set to a rotary position that follows the working line. The jigsaw can therefore be operated semi-autonomously, since it remains for the user merely to assume the feed and to support the reaction forces, whereas the guiding along the working line is assumed by the detection system.
A prerequisite for such a semi-autonomous working mode is the perfect acquisition of the working line by means of a sensor system, belonging to the detection system and realized, if appropriate, as a camera, in the region of a detection field that comprises the work region of the saw blade on the workpiece and to which the sensor system is aligned.
Although this region is usually illuminated by a lighting unit that, like the sensor system, is disposed on the machine side, a working line constituted by an applied marking, for example a pencil mark, cannot always be perfectly recognized and unambiguously distinguished from the background, particularly in the case of strongly patterned or grained surfaces of the respective workpiece.